This invention relates to a method for preparing rare earth permanent magnets to be exposed to refrigerants and/or lubricants for an extended period of time, and especially useful in high efficiency motors.
Owing to their magnetic properties and economy, rare earth permanent magnets are utilized in many areas of electric and electronic equipment. The production of rare earth permanent magnets is rapidly increasing in these years. As compared with rare earth cobalt magnets, rare earth permanent magnets are advantageous in that neodymium as the predominant element is present in more plenty than samarium, the raw material cost is low because of the relatively low content of cobalt, and their magnetic properties substantially surpass those of rare earth cobalt magnets. The rare earth permanent magnets now find use not only in small-size magnetic circuits where rare earth cobalt magnets have been used, but also in areas where hard ferrite and electromagnets have been used. Also in the field of motors for use in compressors in air conditioners and refrigerators, transition from prior art induction motors and synchronous motors using ferrite magnets to DC brushless motors using rare earth magnets is in progress for the purpose of increasing energy efficiency for reducing the power consumption.
Rxe2x80x94Fexe2x80x94B permanent magnets have the drawback that they are readily oxidized in humid air within a short time since they contain rare earth elements and iron as main components. When Rxe2x80x94Fexe2x80x94B magnets are incorporated in magnetic circuits, oxidative corrosion can reduce the output of magnetic circuits and generate rust with which the surrounding equipment is contaminated. Therefore, rare earth magnets are generally surface treated prior to use. The surface treatment on rare earth magnets includes electroplating, electroless plating, aluminum-ion plating, and various coating techniques.
To find use in air conditioner compressor motors and industrial motors to operate in refrigerant, lubricant or mixed systems, the rare earth permanent magnets are required to be corrosion resistant under high pressure and high temperature conditions in the refrigerant and refrigerating machine oil mixed system.
For example, JP-A 11-150930 discloses the use of non-surface-treated rare earth magnet as the core of the rotor in a refrigerating compressor. However, the combination of HFC refrigerant with an ether or ester base refrigerating machine oil can detract from the magnetic properties of the magnet incorporated in the system during a long term of operation at high temperature.
Also in automotive motors to be operated while kept immersed in lubricants, corrosion reaction will take place between the magnet and the lubricant, detracting from magnetic properties.
Then in these applications, it must be contemplated to carry out any of the above-mentioned surface treatments. However, the Al-ion plating technique is expensive and industrially inexpedient. Coating is unacceptable because of reaction with solvents and oil. The plating technique has the problem of instability at high temperature, as demonstrated by stripping of a plated coating at the temperature of shrinkage fit between the rotor and the shaft. It is difficult to industrially apply the plating surface treatment to large size magnets, yielding many undesirably plated parts.
As discussed above, rare earth permanent magnets for use in high efficiency motors are exposed to the refrigerants and/or lubricants at high temperature and high pressure for an extended period of time and will detract from their magnetic properties due to reaction or corrosion therewith.
An object of the invention is to provide a method for preparing a rare earth permanent magnet having improved stability, corrosion resistance and hydrogen barrier property under rigorous conditions as discussed above.
In one embodiment, the invention provides a method for preparing a rare earth permanent magnet to be exposed to a refrigerant and/or lubricant for an extended period of time, comprising the steps of casting an alloy based on R, T and B, wherein R is neodymium or a combination of neodymium with one or more rare earth elements, T is iron or a mixture of iron and cobalt, and B is boron, said alloy consisting essentially of 17 to 33.5% by weight of neodymium, 26.8 to 33.5% by weight of the entire R (inclusive of neodymium), 0.78 to 1.25% by weight of B, 0.05 to 3.5% by weight of at least one element selected from the group consisting of Ni, Ga, Zr, Nb, Hf, Ta, Mn, Sn, Mo, Zn, Pb, Sb, Al, Si, V, Cr, Ti, Cu, Ca and Mg, the balance being T and incidental impurities; crushing the alloy in an oxygen-free atmosphere of argon, nitrogen or vacuum, followed by comminution, compacting under a magnetic field, sintering and aging, thereby yielding a sintered magnet having an oxygen concentration of up to 0.8% by weight, and magnetic properties including a residual flux density Br of 12.0 to 15.2 kG and a coercive force iHc of 9 to 35 kOe; cutting and/or polishing the sintered magnet to give a finished surface; and heat treating the sintered magnet in an argon, nitrogen or low-pressure vacuum atmosphere having an oxygen partial pressure of 10xe2x88x926 to 100 torr for 10 minutes to 10 hours.
Another embodiment provides a method for preparing a rare earth permanent magnet to be exposed to a refrigerant and/or lubricant for an extended period of time, comprising the steps of furnishing a mother alloy based on R, T and B, wherein R is neodymium or a combination of neodymium with one or more rare earth elements, T is iron or a mixture of iron and cobalt, and B is boron, said mother alloy consisting essentially of 17 to 33.5% by weight of neodymium, 26.8 to 33.5% by weight of the entire R (inclusive of neodymium), 0.78 to 1.25% by weight of B, 0.05 to 3.5% by weight of at least one element selected from the group consisting of Ni, Ga, Zr, Nb, Hf, Ta, Mn, Sn, Mo, Zn, Pb, Sb, Al, Si, V, Cr, Ti, Cu, Ca and Mg, the balance being T and incidental impurities, and an auxiliary alloy consisting essentially of 28 to 70% by weight of Rxe2x80x2 wherein Rxe2x80x2 is at least one rare earth element, 0 to 1.5% by weight of B, 0.05 to 10% by weight of at least one element selected from the group consisting of Ni, Ga, Zr, Nb, Hf, Ta, Mo, Al, Si, V, Cr, Ti and Cu, the balance being a mixture of iron and cobalt and incidental impurities; hydriding and crushing the mother alloy in an oxygen-free atmosphere of argon, nitrogen or vacuum; mixing 85 to 99% by weight of the crushed mother alloy with 1 to 15% by weight of the auxiliary alloy, followed by comminution, compacting under a magnetic field, sintering and aging, thereby yielding a sintered magnet having an oxygen concentration of up to 0.8% by weight, and magnetic properties including a residual flux density Br of 12.0 to 15.2 kG and a coercive force iHc of 9 to 35 kOe; cutting and/or polishing the magnet to give a finished surface; and heat treating the magnet in an argon, nitrogen or low-pressure vacuum atmosphere having an oxygen partial pressure of 10xe2x88x926 to 100 torr for 10 minutes to 10 hours.
Regarding a rare earth magnet which is used in various high efficiency motors (complying with the revised energy saving regulation enacted in Japan) and exposed to HFC alternative refrigerant and/or lubricant under operating conditions for an extended period of time, the inventor has found that corrosion resistance is improved by heat treating the magnet, which has been surface finished as mentioned above, in an argon, nitrogen or low-pressure vacuum atmosphere having an oxygen partial pressure of 10xe2x88x926 to 100 torr, and preferably at a temperature of 200 to 1,100xc2x0 C., for 10 minutes to 10 hours.